1. Field of the Invention
The present invention relates to towed acoustic transmitters which make it possible to transmit acoustic signals in seawater which are intended in particular to be received, after reflection on a detected obstacle, by a linear acoustic receiving array which is itself towed astern of this transmitter.
2. Description of the Related Art
It is known to tow behind a boat an underwater vehicle which may contain various payloads such as a sonar. This vehicle sails astern of the towboat at a submersion determined for example by flaps which are manoeuvred appropriately so as to maintain this submersion at the desired value.
A particular case consists, as represented in FIG. 1, in towing behind a hauling boat 101, with the aid of a towing cable 102, a vehicle 103 essentially comprising an acoustic transmitter which makes it possible to sweep the underwater volume with sound. The return echoes obtained through this sound-sweeping are received by an acoustic linear array 105, which is itself towed by a cable 104 fastened astern of the underwater vehicle 103. This kind of device being intended to operate at relatively low frequencies, of the order of a kHz, the transmitter contained in the vehicle 103 is therefore relatively voluminous. To obtain the directivity characteristics which are generally desired, in particular in the vertical plane so as to combat the phenomena of reverberation, this acoustic transmitter is generally of elongate shape, with a height of around 4 to 5 times its thickness as well as its width.
In order to be able to maintain these directivity characteristics, it is of course necessary for the transmitting array, and hence the body of the vehicle to which it is generally rigidly attached, to move along a rectilinear trajectory, at the very least while the boat is not turning, and to maintain a constant trim and constant heading, that is to say in practice for the body to remain vertical and oriented towards the boat. To achieve this, use is generally made of a structure such as represented in FIG. 2. The body 203 of the vehicle, which has substantially the shape of a column with elliptical cross-section of fairly large aspect ratio, is hauled by the cable 102, to which it is joined by a rigid stirrup 202, whose points of fixing to the vehicle are fixed in an articulated manner at the location of the centre of drag of this body. In this way this stirrup ensures good roll stability of the hauled vehicle. The cable 104 for towing the linear array 105 is itself tied to the rear of the body, at a spot situated in the horizontal plane passing through this centre of drag. To ensure the vertical stability of the vehicle, the distribution of the masses inside the body is contrived in such a way that the centre of gravity 101 of the latter is located well beneath the articulation between the stirrup 202 and the body 203. The yaw stability originates in part from the action of the stirrup 202 and it is improved by using vertical fins 205 fixed to the top and bottom of the body.
Such a device operates relatively well, at the very least while the height/width ratio of the body of the vehicle is not too large and it is not sought to submerge it too deeply.
Now, the current tendency consists in seeking deeper and deeper submersions both in respect of the transmitter and in respect of the receiving array associated therewith, essentially so as to increase the detection range. Under these conditions, and since the length of the hauling cable 102 cannot be permitted to be excessively lengthened, for large depths an inclination of the stirrup 202 with respect to the vertical of the body of the vehicle is obtained which is itself large. Bringing the stirrup closer to this vertical tends to increase the instability of the body of the towed vehicle considerably.
Furthermore, in order to have simultaneously a higher array gain and lower electrical consumption, it is also sought to use much taller transmitters than those already known. The vehicles comprising these transmitters then have substantially the shape of a vertical wing of large height/width ratio. Such vehicles have very poor stability in yaw and in the event of the manoeuvring of the hauling boat, thus tending to make them drift from a rectilinear trajectory. Furthermore, as soon as the speed exceeds about ten knots the appearance of an effect of the "kite" type is observed. Under this effect the body pulls to one side or the other and tends to lie down, and this causes it to resurface beyond a certain speed since the hydrodynamic lift then becomes very large compared with this body's weight in the water.
All these effects are unacceptable during operational use and it is not conceivable to limit the operational capacity of this system as a function of the circumstances encountered, for example by reducing the speed.
It has been attempted to improve this system by fitting it with a tail assembly, articulated to the same point of articulation as the stirrup 202 and which makes it possible to stabilize this undesirable motion to a certain extent. The cable 104 enabling the linear array to be towed is then fastened to the rear of this tail assembly. Unfortunately, when extracting from the water during the operations for recovering the whole the tail assembly tends to rise up, and in order to avoid this undesirable effect it is expedient to motorize the articulation between the tail assembly and the body, so as to align the latter with the tail assembly before extraction from the water. This motorization naturally complicates the device and increases its cost. It is furthermore only relatively effective and it is observed that the placing of such a body into the water and its recovery present major difficulties. In fact, when part of the body begins to emerge, the drag of the submerged part becomes predominant and tends to make the device as a whole tip up. The motions can then no longer be controlled, all the more so since disturbances due to the wake from the boat and to the wash from the propellers are added to the disturbances due to the transit at the air/water interface.
Furthermore the location for storing this device in the towboat outside of operational periods is of limited dimensions. In order to be able to stow the vehicle in this location, it is then expedient to install a system for tipping the vehicle up at the time of anchorage or recovery, so as to be able to stow it in a position which is compatible with the limited height available in the storage location. The means to be implemented are heavy and expensive and their use is risky and dangerous.